What is the AGI limit above which each of the following taxpayers would not be eligible to receive a credit for the elderly or the disabled?

$\backslash$table$[[,\backslash$table$[[$AGI Upper$],[$Limit$]]],[\backslash$table$[[$a$.$ A single taxpayer eligible for the credit who receives $$2,700$ of nontaxable social$],[$security benefits$]],],[\backslash$table$[[$b$.$ Taxpayers filing a joint return for which one taxpayer is eligible for the credit and the$],[$taxpayers have received no social security benefits.$]],],[\backslash$table$[[$c$.$ Taxpayers filing a joint return, and both are eligible for the credit and received $$4,700$